This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-104859, filed Apr. 3, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device such as a diode or a transistor which utilizes SiGe.
2. Description of the Related Art
An SiGe mixed crystal film is used to form an electronic device in which the SiGe mixed crystal film is joined to Si, for example, a device which has a structure in which n-type Si, p-type SiGe, and n-type Si are sequentially joined to each other, that is, a heterojunction bipolar transistor. The transistor exhibits excellent high-frequency characteristics superior to a transistor with a structure using Si alone. Accordingly, the SiGe mixed crystal film is lately becoming widespread in integrated circuits for high frequency. The present inventors disclosed the following fact in the specification and the drawings of Jpn. Pat. Appln. No. 2000-044306 (hereinafter referred to as the prior patent application). That is, in a diode in which p-type SiGe is joined to n-type Si, recovery time required when the application of bias is changed from the normal direction to the reverse direction is shorter than that of a conventional Si diode, so that a high-speed operation can be realized.
As for the heterojunction transistor or diode utilizing the SiGe mixed crystal film having such characteristics, from the viewpoints of improving the yield and increasing the range of uses, manufacturers strongly demand that breakdown voltage characteristics rise. The diode structure near to a portion where a breakdown voltage is applied, that is, the structure in which p-type SiGe is joined to n-type Si, or the structure in which n-type SiGe is joined to p-type Si will be referred to as an SiGe/Si diode. In the SiGe/Si diode, a leakage current becomes a problem when reverse bias is applied on the pn junction boundary (interface)
A leakage current generated in a conventional SiGe/Si diode will now be described with reference to FIG. 1. FIG. 1 schematically shows a leakage current 17 generated when reverse bias is applied to an SiGe/Si diode 10. When reverse bias is applied to the diode 10, as shown in the diagram, a depletion layer 13 is formed in an area including a pn junction boundary 18 and an electric field concentrates on the depletion layer 13. Portions 16 where the diode is exposed exist on the pn junction boundary 18. A depletion layer 13a in each exposed portion 16 tends to be narrower than the depletion layer 13 excluding the layer 13a. 
Consequently, the degree of electric field concentration rises in the exposed portion 16 and the leakage current 17 generated in the diode remarkably depends on the substance characteristics of the exposed portion 16.
Specifically, the substance characteristics of the exposed portion 16 controlling the leakage current 17 include a crystal defect, air discharge, and impurities in the exposed portion 16. Among them, the crystal defect depends on a process of manufacturing semiconductor layers. Since attention is sufficiently paid to the quality management of the semiconductor manufacturing process so as to inhibit a crystal defect as much as possible, the crystal defect does not generally grow so severely that it cannot be ignored by itself.
However, effective countermeasure against impurities is not found so far, though the above two disadvantages have been effectively inhibited. As elements serving as impurities causing the leakage current 17, metals such as Na, K, Fe, and Au and impurities such as hydrocarbon and the like which exist in the atmosphere or which are adhered upon cleaning with water are mentioned. When the exposed portion 16 is oxidized to form Ge oxide on the surface, the oxide also causes the leakage current 17. Accordingly, to reduce the leakage current 17, a process of suppressing crystal defect, suppressing air discharge, and sufficiently reducing harmful impurities causing the leakage current 17 is required in the exposed portion 16.
In the conventional diode constructed by Si as a whole, generally, the exposed portion 16 on the pn junction boundary is subjected to a thermal oxidization treatment. That is, the surface of the diode is oxidized in an atmosphere of oxygen or water vapor at a high temperature of 900xc2x0 C. or more. When the thermal oxidization treatment is performed, Si of the exposed portion 16 is oxidized to be insulated, so that the leakage current 17 is reduced. Although the thermal oxidization method is not effective against metal impurities, it is effective in an Si-based device. The thermal oxidization method is often used in the Si-based device so far.
When the conventional thermal oxidization method is applied to the SiGe/Si diode as it is, Ge is segregated on the interface between SiGe and an oxidized layer formed on the surface of SiGe. The segregation causes the leakage current 17. Ge oxide (GeO, GeO2) has high conductive properties. The Ge oxide itself also becomes a cause of the leakage current 17. Accordingly, to perform the thermal oxidization method to SiGe, searches and researches with much labor regarding oxidizing conditions that Ge is not segregated are needed. Effective means is not found so far and the problem is left outstanding.
The problems of the foregoing conventional technique are concerned with the heterojunction boundary of the SiGe/Si diode. The junction boundary of an SiGe/SiGe diode also has the similar problems.
It is an object of the present invention to provide a method for manufacturing a semiconductor device in which a leakage current is not generated on an SiGe/Si heterojunction boundary or an SiGe/SiGe junction boundary, particularly, an exposed portion thereof.
According to a first aspect of the present invention, there is provided a method for manufacturing a semiconductor device having a junction boundary where SiGe of a first conductivity type and Si or SiGe of a second conductivity type come in contact with each other, the method comprising the steps of: cleaning a portion, where the junction boundary is exposed on the surface, with a first solution containing hydrofluoric acid; and cleaning the portion with a second solution containing sulfuric acid.
When an exposed portion on the surface of the junction boundary of an SiGe/Si diode is left in the atmosphere, it is oxidized spontaneously. In this instance, as impurities, adsorption of impurities (such as hydrocarbon and the like) in the atmosphere and mixing of metal (Na, K) ions given on contact with the operator""s bare hands are expected, and furthermore, an impurity such as Ge oxide (GeO2), which is formed by oxidizing Ge, is expected. The impurities cause a leakage current, resulting in deterioration of breakdown voltage characteristics of the semiconductor device.
The first aspect of the present invention is concerned with the method for effectively eliminating impurities on the surface layer. According to the first aspect of the invention, first immersing the SiGe/Si diode in the first solution containing hydrofluoric acid eliminates oxide formed on the exposed portion. When the oxidized layer has a thickness of several microns, it can be easily eliminated so long as time to immerse in the hydrofluoric acid solution is changed. Due to the cleaning treatment, the junction boundary is terminated with hydrogen on the surface of the exposed portion. In the process, however, hydrocarbon and metal impurities are not eliminated.
Subsequently, when the SiGe/Si diode is immersed in the second solution (solution containing sulfuric acid), the metal impurities and hydrocarbon dissolve in the solution to be removed from the surface layer. At that time, the surface layer is oxidized at a thickness of about 1 nm (10xc3x85). In this instance, SiO2 alone is formed and Ge oxide (GeO2) is not formed. A Ge atom on the surface layer is oxidized due to influences of sulfuric acid. Since Ge oxide dissolves in the sulfuric acid solution, GeO2 is not left on the surface layer. On the surface layer obtained in this case, impurities such as metal, hydrocarbon, and Ge oxide are reduced. In addition, Si oxide is formed thin on the surface layer. The Si oxide is inert to impurities given from the outside and functions to inhibit the adsorption of impurities, which will occur later.
According to the first aspect of the invention, the treatment with hydrofluoric acid and the treatment with sulfuric acid are sequentially performed. When either of them is not performed, advantages of the present invention are not achieved. In a case where the treatment with hydrofluoric acid is not performed, when a spontaneously oxidized film on the surface layer is thick, GeO2 in the spontaneously oxidized film is not sufficiently eliminated and metal impurities contained in the spontaneously oxidized film cannot be eliminated. On the other hand, when the treatment with sulfuric acid is not performed, metal impurities and hydrocarbon, which cannot be removed by the treatment with hydrofluoric acid, are remained. When the treatment with hydrofluoric acid and the treatment with sulfuric acid are performed in inverse order, the surface is terminated with hydrogen. Consequently, as compared with the diode according to the present invention, in which the surface is terminated with the oxidized film, advantages for inhibiting the adsorption of impurities, which will occur later, are weakened.
According to a second aspect of the present invention, there is provided a method for manufacturing a semiconductor device having a junction boundary where SiGe of a first conductivity type and Si or SiGe of a second conductivity type come in contact with each other, the method comprising the steps of: etching the surface of a portion, where the junction boundary is exposed, with a first solution; cleaning the portion with a second solution containing hydrofluoric acid; cleaning the portion with a third solution containing sulfuric acid; and coating the portion, where the junction boundary is exposed on the surface, with an insulating material.
As mentioned above, 1) a crystal defect, 2) air discharge, and 3) impurities in the exposed portion on the surface of the junction boundary become causes of the leakage current generated when reverse bias is applied to the SiGe/Si diode.
According to the second aspect of the invention, a treatment of eliminating 1) the crystal defect and a treatment of eliminating 2) the air discharge are added to the method according to the first aspect of the invention. According to the second aspect of the invention, there is provided a diode with lower leakage current.
Specifically, to eliminate the crystal defect on the semiconductor surface, the surface layer is etched with a chemical etching agent as a first solution, e.g., an aqueous alkaline solution such as KOH or an aqueous mixed solution of hydrofluoric acid and sulfuric acid. Consequently, the crystal defect caused by scratches or surface damage due to a treatment with plasmas can be eliminated. After that, to eliminate impurities, the diode is immersed in the solution containing hydrofluoric acid (second solution) and the solution containing sulfuric acid (third solution), which are used in the first aspect of the invention, in the order to be washed. Furthermore, the surface layer of the diode is coated with an insulating material such as gelled silicone (for example, silicone gel made by Shinetsu Silicone Co.) to prevent the surface from coming in contact with the air. Consequently, the occurrence of air discharge is inhibited. Accordingly, the SiGe/Si diode with slight performance degradation due to the leakage current can be obtained by combining the above three treatments.
Although the above treatments have been described with respect to the example of the SiGe/Si diode, the similar advantages can be also achieved in the SiGe/SiGe diode.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.